This invention relates generally to seabed plows and more particularly concerns a deep digging over-the-stern trenching plow with instrumentation for assessing the protective capabilities of a seabed trench.
The present practices and equipment, typically requiring cranes and associated heavy equipment and structures, used to release and retrieve a plow from a vessel into the sea and from the sea onto the vessel typically limit the weight of the plow to a maximum of approximately 20 tons. The trenching depth and strength of known plows are compromised accordingly.
The depth achievable in the first trenching pass of these known 20 ton trenching plows is at best 1.4 meters. Deeper trenches can be dug by multiple passes, but the deeper the trench and the greater the number of passes, the greater the forces applied to the limited strength plow. Therefore, even when multiple passes of known trenching plows are run, a trench depth of approximately 2.7 meters is the most that can be expected. But, in many applications, trenches three meters deep may be insufficient to protect their buried contents. Consider, for example, the impact forces that might be applied to a pipeline buried in a trench located in an iceberg zone.
On the other hand, there is a plow weighing 200 tons that requires use of an A-frame or crane for launch and retrieval and can achieve a first pass depth of 2.0 meters and a maximum total depth of 2.7 meters. The maximum depth of 2.7 meters is dictated because the configuration required of the plow for launch and retrieval by A-frame or crane does not afford a plow of sufficient strength to withstand the forces that will be incurred in excavating a trench greater than 3.0 meters in depth, regardless of the number of passes used for the purpose.
Assuming that a suitable seabed trench can be excavated, the capability of the trench to protect pipelines, cables and other objects laid or buried in a seabed trench is a foremost concern. For example, the likelihood that damage may be caused by icebergs and other undersea objects drifting or otherwise moving in the vicinity of the trench is a function of the composition of the soil in which the object is laid or buried and the depth at which the object is laid or buried in the soil.
Plow tip sensors are presently used to measure the shearing force applied by the tip of the plow to the seabed. Load cells are also presently used to measure the total tow force applied to the trenching plow. It is presently understood that the difference between the measured shearing and total tow forces will be generally indicative of the non-tip forces applied to the plow. Such information is useful to understanding the orientation of and the forces applied to the plow during the trenching process but does not afford an assessment of the protective capabilities of a trench.
The assessment is complicated because the composition of the soil may change considerably along the path of the trench and the depth of the trench along its path may vary somewhat from the depth expected from a given design and adjustable configuration of the trenching plow.
It is, therefore, an object of this invention to provide a trenching plow capable of digging trenches deeper than can be dug by known trenching plows. It is also an object of this invention to provide a method for over-the-stern release and retrieval of a deep digging trenching plow from a vessel into the sea and from the sea onto the vessel. It is another object of this invention to provide a method and instrumentation for assessing, on a real-time basis, the ability of a trench to protect objects laid or buried in the trench from damage by the impact of external objects.